| Title | Review of Geothermal Drilling Operations -Toward More Effective Operations |
|---|---|
| Authors | Rowley, J. C. |
| Year | 1988 |
| Conference | Japan International Geothermal Symposium |
| Keywords | |
| Abstract | Drilling practices and operations for geothermal development have usually been adaptations of oil field or mining experience and equipment. The subsurface conditions. rocks. and environment in geothermal reservoirs are quite different. Mining exploration drilling equipment. primarily diamond core drills. have been a primary method used for geothermal exploration. and this equipment has been used very successfully worldwide. These shallow. small diameter. core hole rigs are optimized for the very hard: abrasive. fractured rocks encountered in geothermal areas. These core holes are intended primarily for thermal gradient determinations and for fluid sampling and monitoring [1.2]. The adaptation of production well drilling from the oil field has been less successful and the resulting high drilling costs remain as one impediment to more rapid geothermal development in the United States and elsewhere in the world. Drilling costs are increased over oil field drilling at comparable depths due to the harder. abrasive rocks and extreme temperature encountered in most geothermal reservoirs. Drilling costs are a significant factor in a geothermal electric power plant development project[3]. and range from 40 to 60% of total project costs in the United States; and can be an even larger fraction in other locations in the world. Therefore drilling cost reduction and optimization of production drilling operations can have a significant influence on the rate of geothermal development progress. There are additional significant differences between oil field and geothermal well drilling strategies and risks. The usual oil field practice concentrates drilling activities early in the field development and the production rate declines over the production history. a condition acceptable to the petroleum market. Initial geothermal field drilling usually includes only enough wells early in the project to supply the first power plant to be constructed[4]. and the pressure and enthalpy supply rate must be maintained constant to the plant. A declining production cannot .be tolerated. so a few extra wells (usually with about 20%) are drilled initially. Experience indicates that about one new well (or a standby well activated) must be drilled per year per 100 MW(e) of power capacity to sustain production and replace damaged and aging wells. In the US. most geothermal drilling equipment methods. and operations have been adopted and adapted from the petroleum industry. But. because the average depth of geothermal wells (2.1 km in the US) is twice that of the average oil well. and is drilled in very hard. abrasive. crystalline rocks at high temperatures. geothermal drilling costs are often 2 to 4 times those of comparable oil wells[5]. As a result of these factors; however. many of the problems causing the higher costs have technological solutions[3]. Another major element in high geothermal drilling costs is the large variations in subsurface geology. It has often been observed that geologic changes within the reservoirs on a scale less than the well spacing are common. Reduction of increased costs due to extreme subsurface complexities can be achieved by close on-site support and coordination of the drilling operations by the project geologists. geochemists. and gElophysicists. Projection of the subsurface conditions for each new well. detailed well planning and review. and close on-line monitoring of drilling are the essential functions of this team of professionals. if cost? are to be reduced. There are two primary areas of geothermal drilling where appropriate improvements will result in major cost reductions. These are lost circulation control and enhanced drill bit life. The remainder of this paper will focus on these two topics. |